1. Field of the Invention
This invention relates to improved magnetic bubble domain chips and processes for making these chips using only a single critical masking step, and is more particularly directed to a magnetic bubble domain chip comprising two levels of metallurgy, where one level has apertures (or recesses) therein filled with an electrically insulating material. The insulating material is used in those areas of the chip where the presence of a conductor between a bubble device (magnetic element) and the underlying bubble film would be detrimental to device operation.
2. Description of the Prior Art
Magnetic bubble domain devices are well known in the art and commercial products using this technology are now available. The processing steps used to make a complete magnetic bubble domain chip (having the functions of read, write, storage, transfer, and annihilation) are well known in the art. In one type of processing, called double level metallurgy (DLM), a plurality of masking steps is used where more than one masking step requires a critical alignment. Since alignment is not difficult at the dimensions used for making bubble devices using bubbles of 3 microns diameter and larger, such processes are commonly in use. However, for bubble domains having diameters less than about 3 microns, mask alignment becomes more difficult. For very small bubble domains (of about 2 microns and less in diameter) the concept of single level masking (SLM) has been proposed as a desirable technique.
In SLM processes, more than one masking level may be involved; however, no critical alignment is required and only one level has fine line features. For this reason, such a technique is desirable for the fabrication of ultra-high density bubble devices and permits the use of electron beam, x-ray, or deep UV conformable mask lithography. Further, in DLM processes, there is often the problem of assuring a smooth transition of a NiFe overlayer over a conductor. Also, it is often necessary in such processes to provide an insulator between the NiFe layer and the conductor.
SLM processes largely avoid the problems described in the previous paragraph. Early SLM processes used bubble device designs in which the bubble propagation elements, sensors, and current-carrying lines were all made of NiFe. In order to reduce the resistance and hence the voltage drop along the lines, it was suggested to use a layered structure comprising a bottom layer of NiFe and an overlayer of gold (Au). Although this structure has considerably lower resistance per unit length of line than a structure using only NiFe, it does require higher switch currents for bubble transfer because the NiFe under the conducting Au layer shielded the current-produced magnetic flux so that it did not penetrate the bubble material lying below the conductor. For this reason, designs of bubble devices have been presented in which the current carrying conductor is located below the NiFe layer, in the areas of the magnetic chip where current carrying conductors are used to determine the paths followed by bubble domains. This structure has the advantage of lowering the resistance of the lines, and also requires less current than the approaches using only NiFe or NiFe--first, Au--second. This is because the current is confined closer to the bubble domain material, and because the NiFe on top of the Au acts as a magnetic yoke which increases the fields in the bubble material.
From a circuit design standpoint, it is known that the presence of the current-carrying conductor beneath the magnetic propagation elements does not adversely affect their function. However, it is desirable to have the magnetic elements used for bubble domain generation and bubble domain sensing close to the magnetic bubble material. If these elements are shielded from the bubble material by a thick current-carrying conductor, their operational margins are severely restricted. Also, the magnetic sensing element cannot be electrically shunted by a conductor. Thus, the problem is to devise a magnetic chip and a SLM process for making it where the conductive layer lies below the magnetic layer used to move the bubble domains, except in the regions of the magnetic chip used for bubble domain generation and bubble domain sensing. Further, it is often desirable to provide a magnetic chip which is essentially planar and a process which achieves planarization, i.e., a process in which each overlayer lies in a single plane substantially throughout the magnetic chip.
Accordingly, it is a primary object of the present invention to provide an improved bubble domain chip and a SLM process for making it.
It is another object of the present invention to provide a new bubble domain chip and a process for making it wherein an insulating pedestal is used to adjust the height of the bubble generator and bubble detector with respect to the underlying bubble material.
It is yet another object of the present invention to provide a new bubble domain chip and a SLM process for making it using conductor-first metallurgy and only one mask alignment.
It is another object of the present invention to provide an improved bubble domain chip and a SLM process for making it, characterized by the use of an insulating pedestal in the regions of the sensor and the generator to insure the planarity of the generator and sensor with respect to an underlying conductor layer.
It is yet another object of the present invention to provide a new bubble domain chip and a SLM process for making it, where the operational margins of the devices in the complete chip are not adversely affected by the process.
It is another object of the present invention to provide a novel bubble domain chip characterized by bi-level metallurgy including at least one electrically conductive non-magnetic layer and one magnetic layer.
It is a further object to provide a planar magnetic bubble domain chip which can be made by a SLM process and which is particularly suitable for manipulating stable bubble domains having diameters of about 20,000 angstroms (two microns) and less.